Help about my speedmaster 321, made in USA?

There are no 23m Speedmasters AFAIK, they jump from 22m to 24m, at least I can't recall seeing one. The case isn't marked as Star or anyone else American, it is a regular HF -67 case, ergo I too think this is a movement transplant from another chrono. Note the -67 cased watches were mainly produced in early 1968. I would suggest that the case is younger than the movement. It may be the picture, but the dial looks to have been washed of lume.

Or it’s a service movement.

in any event it has little interest to a speedmaster collector, except as a breaker

A Speedmaster Professional made in August 1966 is reference 105.012-65. Not even a 105.012-66, and for sure not a 145.012-67.
So regardless if this was originally a Speedy movement (which it wasn't). This is a parts watch even if it was...